Electronic control devices mounted on vehicles have generally been so constructed that an internal microcomputer and peripheral circuits operate on the same electric power source voltage (for example 5 V).
In recent years, however, accompanying the trend toward higher performance required for microcomputers, internal cores (e.g., CPUs and memories) have now been operated at an electric power source voltage (e.g., 3.3 V) lower than the customarily employed voltage to accomplish a high-speed operation. However, input/output circuits in the microcomputer for exchanging signals with external circuits and peripheral circuits of the microcomputer, still use inexpensive parts that operate on the conventional electric power source voltage.
Therefore, it has been urged to provide an electric power source device capable of providing plural kinds of electric power source voltages to supply the conventional electric power source voltage to the input/output circuits in the microcomputer and to the peripheral circuits, as well as to supply a low electric power source voltage to the cores in the microcomputer.
As an electric power source device of this kind is so constructed as to supply plural kinds of electric power source voltages to an external unit by combining switching regulators and series regulators in a plurality of sets on the integrated circuit (U.S. Pat. No. 6,424,128 B1).
U.S. Pat. No. 6,424,128 B1 proposes an electric power source circuit capable of supplying two kinds of electric power source voltages, having a switching regulator that meets a large current consumption mode in a state where the microcomputer is active and a series regulator that meets a small current consumption mode in a standby state, to supply a low voltage to the microcomputer. The electric power source circuit further has a series regulator for supplying an electric power source voltage to other circuits.
Further, as the functions of a vehicle-mounted electronic control device are becoming sophisticated year after year, not only the microcomputers are consuming increased currents but also the peripheral circuits are consuming increased currents, too. To meet this tendency, therefore, it can be contrived to replace the series regulator in the electric power source circuit that supplies power to other circuits by a switching regulator capable of supplying a large current with a power loss smaller than that of the series regulator, to produce two kinds of electric power source voltages relying upon the switching regulators. One circuit construction is shown in FIG. 7.
FIG. 7 schematically illustrates a multi-output electric power source device 90 capable of producing two kinds of electric power source voltages by using two switching regulators. This multi-output electric power source device 90 includes two switching regulators. That is, a first switching regulator is provided for producing a constant voltage V4 converted (lowered) from an external electric power source voltage V1 that is input through an input smoothing circuit 91 comprising a coil and a capacitor. The first switching regulator comprises a switching MOSFET (MOS1), a first switching control circuit 70 and an output smoothing circuit 77. A second switching regulator is provided for producing a constant voltage V7 converted (lowered) from the external electric power source voltage V1. The second switching regulator comprises a MOS2, a second switching control circuit 80 and an output smoothing circuit 87.
The switching regulators operate in the same manner but produce different output voltages. Therefore, the first switching regulator will be briefly described below. The input voltage V1 from the external electric power source is applied to the MOS1 through the input smoothing circuit 91 and a current detecting resistor R41. The MOS1 is turned on and off by a control signal (voltage pulse signal) from the first switching control circuit 70, and a pulse-like voltage output from the MOS1 is converted into a stabilized average voltage through the output smoothing circuit 77 constructed by a flywheel diode, a coil and a capacitor. The average voltage is supplied as a constant voltage output V4 to a core 61a in a microcomputer 61.
The first switching control circuit 70 is a known one that provides a control signal to the MOS1 based on the feedback of the constant voltage output V4. The output voltage V4 that is fed back is divided and is input to a switching regulator control circuit 71. The switching regulator control circuit 71 provides an error signal based on a difference of a divided voltage value and a reference voltage Vr1 from a reference voltage generating circuit 73. The error signal is compared with a triangular wave from a triangular wave generating circuit 74 to determine an on/off duty ratio of the MOS1.
Unlike the devices for use commercially, the electric power source device for vehicle-mounted electronic control devices must have self-protecting function for maintaining safety against abnormal conditions in the electric power source device (over-voltage, over-current in the electric power source path, over-heating of switching elements, etc.) caused by external factors such as fluctuation in the external electric power source at the receiving end, and short-circuit of the wiring, over-load, etc. on the output side.
Therefore, the device is so constructed as to detect an over-current in the current-carrying path by detecting a current in the current-carrying path of from the input voltage side of the external electric power source to the microcomputer 61 to which the electric power source is to be supplied relying upon the current detecting resistor R41 and a differential amplifier 75, and by inputting the current to the switching regulator control circuit 71, as well as to detect an over-voltage by dividing the input voltage by the voltage-dividing resistors R72, R73 and comparing the divided voltage with a reference potential by using a comparator 76. Further, an over-heat detector circuit 72 for detecting the over-heating of the MOS1.
When any one of over-current, over-voltage or over-heat is detected, the switching regulator control circuit 71 so controls the first switching regulator that no constant voltage output V4 is produced to an external unit. The second switching regulator operates in quite the same manner as to produce the constant voltage output V7 which is fed to an I/O port 61b in the microcomputer 61.
With the multi-output electric power source device 90 shown in FIG. 7, however, different voltages can be supplied to the core 61a and to the I/O port 61b relying upon the two switching regulators. In case any of the above abnormal condition is detected in either one of the switching regulators, however, the output is no longer produced from the defective switching regulator, and the operation of the microcomputer 61 loses operation stability.
In, for example, the first switching regulator, a current of a magnitude (hereinafter referred to as “half-short”) that is smaller than a current that can be determined to be over-current may continue to flow through the current-carrying path from the input side to the output side, and over-heating may be detected by the over-heat detector circuit 72. In this case, the switching regulator control circuit 71 discontinues the output of constant voltage V4. Then, the core 61a ceases to operate while the second switching regulator continues to produce the constant voltage V7, and the I/O port 61b continues to operate.
When the I/O port 61b continues to operate despite the core 61a has ceased to operate, the I/O port 61b may produce an unexpected signal to an external unit to cause error in the operation of various electronic loads connected to be controlled by the microcomputer 61.
FIG. 8 illustrates the operations of MOS1 and MOS2 in the multi-output electric power source device 90 of FIG. 7, voltages at various portions and the operation condition of the microcomputer 61. As shown in FIG. 8, when the multi-output electric power source device 90 starts operating (i.e., when the switching regulators start operating), the MOS1 and MOS2 commences the switching operation to produce constant voltages V4 and V7, and the electric power is supplied to the microcomputer 61. If, for example, the output side of the first switching regulator is placed in a half-short state causing the MOS1 to be over-heated and if this is detected by the over-heat detector circuit 72, the MOS1 discontinues the switching operation and the constant voltage output V4 is no longer produced.
Here, the constant voltage output V4 does not readily become zero as shown but gradually decreases due to residual magnetic energy in the coil and the residual electric charge in the capacitor in the output smoothing circuit 77. The core 61a in the microcomputer 61 continues to operate even on a low electric power source voltage in a step where the output voltage V4 is decreasing. The MOS2, on the other hand, continues its normal switching operation even when the output voltage V4 decreases down to a region where the normal operation of the core 61a cannot be guaranteed, and the electric power continues to be supplied to the I/O port 61b. It is, therefore, probable that the I/O port 61b produces an unexpected control signal to control the external electronic loads even temporarily in a state where the output voltage V4 is decreasing.
Not only in the step where the output voltage V4 to the core 61a drops but even after the output voltage V4 has become completely zero and the core 61a no longer operates, it is probable that the I/O port 61b may still produce an unexpected control signal.
Conversely, though not illustrated, in case an abnormal condition in the second switching regulator is detected, the constant voltage output V7 is no longer output to the I/O port 61b. In this case, too, the power supply continues to flow to the core 61a despite the I/O port 61b is no longer operating, and the electric power is wastefully consumed.
In addition to the above problem of protection function, the multi-output electric power source device 90 of FIG. 7 still has a problem in that since switching regulator circuits are simply connected in parallel to constitute a switching regulator of two systems, the circuit itself must be constructed in a very large scale for controlling the electric power source as compared to the construction in which the switching regulator and the series regulator are connected in parallel. Besides,the electric power source device for the vehicle-mounted electronic control devices must be furnished with a circuit of self-protection function which is indispensable for maintaining safety. Accordingly, the circuit scale increases and the cost increases, too.